In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date:    (i) part of common general knowledge; or    (ii) known to be relevant to an attempt to solve any problem with which this specification is concerned.
Whilst the following discussion concerns paving units, their installation and manufacture, in particular, paving units capable of interlocking with adjacent paving units, it is to be understood that the same principles apply to paving units of any shape or colour.
Small element paving comprises a plurality of paving units arranged together to provide a paved surface. Over the last thirty years small element paving has developed from its initial use as a surface for footpaths and other light duty applications to use as an integral structural surface for heavy duty pavements. Heavy duty pavements are used for example at ports, trucking stations, on roads, at bus stops and at airports.
Such structures must be resistant to the movement of individual paving units relative to one another. If a paving unit is separated from its adjacent paving unit, whether it be due to subsidence of the ground on which the paving unit is laid or to a breakdown in the connection between adjacent paving units, it is likely to present a gap or an otherwise uneven surface.
It will be apparent that that is a potentially dangerous situation, as a pedestrian may catch a heel or the toe of a shoe in the gap created, and may trip and fall. Depending on the width of the gap, it may also have adverse consequences for the tyres and wheels and hence the suspension system of trucks and aircraft, and may lead to a catastrophic outcome if any of those elements was to fail.
A dislodged paving unit may also pose a risk to the turbines of aircraft engines if a loose paving unit was drawn into the engine.
Standard paving units have proved to be inefficient as they do not inherently have sufficient ‘interlock’.
The concept of interlock relates to the ability of an individual paving unit to articulate independently of an adjacent unit without being dislodged from the overall paving structure.
Three forms of interlock should ideally be present in a block paving system, namely:                rotational interlock, which is achieved by the presence of edge restraints.        vertical interlock, which is provided by the presence of stable compacted sand between all adjacent units.        horizontal interlock, which is achieved by a combination of laying paving units in a herringbone pattern and by the use of shaped units.These forms of interlock are intended to prevent paving units from ‘creeping’, that is, moving horizontally when a force is applied to the paving unit in a direction which is not perpendicular to an upper face of the unit. It is also intended to resist rotational and vertical movement relative to an adjacent unit.        
Small element paving units rely on the continuous presence of compacted sand between individual paving units to provide the necessary ‘interlock’ to resist horizontal, vertical and/or rotational displacement of the paving unit. This sand also inhibits ingress of water into the underlying pavement structure. Displacement may manifest itself when a substantial weight is repeatedly placed on an individual unit by, for example, continuous trafficking over an individual unit.
Further displacement can occur as a result of changes in the underlying substratum on which the paving is laid.
Unfortunately, the sand which provides the interlock between the adjacent paving units can, over a period of time, be displaced from the joints by various agents of erosion such as jet blast, use of vacuum sweepers, water flow, and degradation and liquefaction of laying course materials, thereby resulting in a loss of interlock.
The loss of sand in the joints negates the interlock which can lead to serious deformation and eventual disintegration of the paving surface as shown in FIGS. 8 to 13.
Earlier attempts to provide further interlock between individual paving units consisted of designing various shapes to increase interlock. This included E-shaped, Y-shaped and L-shaped paving units with a plurality of engagement points for adjacent paving units.
Although these various shapes have been tested, it has been observed that problems still persist as shaped paving units may not provide sufficient interlock for use as part of heavy duty pavements. These paving units are also costly to manufacture and time consuming to lay given their often convoluted design.
Other attempts to provide suitable heavy duty pavements include the adoption of a mechanical locking device involving complementary recesses and protrusions on adjacent paving units. This feature provided a further degree of interlock between adjacent paving units.
However, whilst an apparent improvement, “self-locking” paving units do not readily resist vertical or horizontal movements. The paving units can also be rotated and become uneven should there be a slight deformation in the overall pavement either by external pressure or by changes in the underlying ground. Hence these are consequently unsuitable for heavy duty pavements with constant traffic.
Similarly, these paving units are also unsuitable for airports as jet aircraft engines can provide a suction effect that can result in the displacement of the individual paving units.